Fix an assert in KBK endgame

The endgame king + minor vs king is erroneusly
detected as king + minor vs king + minor

Here the fix is to detect king + minor earlier,
in particular to add these trivial cases to
endgame evaluation functions.

Spotted by Reuven Peleg

bench: 4727133
Annotate an unlikely condition

No functional change.
Merge branch 'master' into search
Rewrite "Eelco unplugged take 2"

bench: 4297814
Eelco unplugged: take 4

Limit excessive pruning at low depth.

This is the tabel of margins for the corrisponding depth

d: 0, fm0: 3, fm1: 3
d: 1, fm0: 4, fm1: 4
d: 2, fm0: 6, fm1: 6
d: 3, fm0: 10, fm1: 7
d: 4, fm0: 15, fm1: 11
d: 5, fm0: 21, fm1: 15
d: 6, fm0: 29, fm1: 21
d: 7, fm0: 37, fm1: 27
d: 8, fm0: 47, fm1: 35
d: 9, fm0: 57, fm1: 42
d: 10, fm0: 68, fm1: 51
d: 11, fm0: 81, fm1: 60
d: 12, fm0: 94, fm1: 70
d: 13, fm0: 108, fm1: 81
d: 14, fm0: 123, fm1: 92
d: 15, fm0: 139, fm1: 104

bench: 4364793